U.S. Pat. No. 2,656,508 to Coulter discloses what is commonly referred to as the "aperture impedance" or the "Coulter" principle for counting and sizing particles. An exemplary arrangement utilizing this principle is shown in FIGS. 1, 3, 5, and 7. Through a small aperture 1, the fluid 4 containing the particles in dilute suspension is aspirated from one electrically insulating vessel 3 into another similar vessel 5. This aperture 1 provides the only path for fluid or electrical communication between the two vessels 3 and 5. One electrode 7 is immersed in the fluid in the first vessel 3, and a second electrode 9 is immersed in the fluid in the other vessel 5. The passage of a particle through the aperture 1 causes a brief change in electrical impedance measured between the two electrodes 7 and 9. The magnitude of the transient resistance change, called a "resistive pulse", is a measure proportional to the size of the particle. Several thousand particles may be measured in a few seconds, and the data may be sorted into classes to provide a distribution histogram showing the number of particles falling into each size range. However, this basic arrangement has suffered drawbacks, and drawbacks in accuracy can be significant. For example, measurement of particle size range is critical for the production of a wide range of products including ceramics; toners; dyes; powders; cement; sugar; pharmaceutical products and photographic materials. Variations in particle size can critically influence both the manufacturing processes and the characteristics of the final product.
There have been many attempts to address the drawbacks associated with this basic design. However, none of these attempts have been entirely successful. These drawbacks have resulted in limitations to the smallest particle that can be measured with a given aperture size, orientation errors, coincidence errors, trajectory errors, and extended sensing zone errors.
For small particles, the electrical and acoustic noise compete with the small resistive pulse signal generated by the particles resulting in low S/N ratio. Therefore, the smallest particle measurable by the aperture impedance principle is typically 2% of the aperture diameter. With very small apertures, such as a sub-micrometer aperture, the lower limit is higher than 2% because the noise floor rises substantially due to the increased resistance. The noise goes as the square root of the aperture resistance and the aperture resistance is inversely proportional to the square of the aperture cross-sectional area. Therefore, as the aperture becomes smaller, the resistance increases and so does the associated noise. Additionally, for the instruments based on this aperture impedance or electrical sensing zone method, in the measurement of small particles, thermal aperture noise continues to exceed all other noise contributions by more than an order of magnitude. Further improvements in the circuitry cannot lead to better resolution.
The prior art embodiment of FIG. 1 does not take into account the shape of the particle and this leads to an inability to obtain important information about the particles and significant particle orientation errors. The electrical response for cylindrical shaped particles measured by this aperture impedance method can be proportional to the size deduced from a calibration using spherical particles. This may be errors as high as 25%. There is a complex relationship between hydrodynamic forces, deformation of particles, aperture dimensions and pressure and therefore it is not possible to relate the characteristics of the pulse to the shape of the particle.
In an attempt to get more information on the particles, prior art designs have simultaneously passed high and low-frequency currents through the aperture. While the use of appropriate filtering techniques can permit detection of both the low frequency resistance and high frequency reactance of the particle traversing the aperture, the interference created between the two separate current sources employed to create the high frequency and the low frequency current within the aperture cannot be eliminated. Any slight change in conditions can cause either, or both of the two frequencies to become de-tuned.
Further, it is known that generally, due to the hydrodynamic focusing in most instruments, elongated particles will be aligned with their elongated axis substantially parallel to the center axis of the orifice. With two particles of equal volume, one being spherical and one being elongated, the spherical particle while passing thorough the orifice will have a greater cross section perpendicular to the current flow than the elongated particle. Hence, the spherical particle will distort the field in such a manner that it will give a greater measured size than the elongated particle, despite their equal volumes.
FIGS. 1 and 2 illustrate the error in the prior art due to the difference in orientation of the particles. Aperture 1 in the insulator 2 establishes the constricted electrical path of external electrodes. Consider a non-spherical particle 8 with its main axis along the aperture axis, and another non-spherical particle 6 with its main axis perpendicular to the aperture axis. The particle 6 with its main axis perpendicular the aperture axis would obstruct the electric field in the aperture 1 significantly more, and would result in a higher peak 10 as compared to the peak 12 of other particle 8 with its main axis along or parallel to the aperture axis. Thus, it is evident that particle size measurements for non-spherical particles can be fairly erroneous.
Another limitation with prior art devices results in certain instruments counting losses of up to 20% due to random coincidences of particles in the orifice. Simultaneous presence of more than one particle in the aperture can occur without detection. The prior art neglects the co-incident pulses most of the time or provides imprecise corrections. Statistical methods are used to compensate for neglecting these pulses. This inherently limits the accuracy of the instrument. FIGS. 3 and 4 illustrate the error in the prior art due to the co-incident presence of particles in the sensing zone. Assume that a second particle 15 enters the sensing zone before a first particle 17 has left the sensing zone. The result is that the pulse 16 due to the first particle 17 is superimposed with the pulse 18 due to the second particle 15 resulting in a much larger pulse 14.
An additional problem in the prior art is due to trajectory errors. This may arise due to non-uniform current density at different cross-sectional locations within the aperture of the instrument. Because of the non-uniform current density, the pulse height of the related shape depends on the path an individual particle takes through the aperture. The current density is significantly higher at the edges of the entrance and exit of the aperture. Also, the electrolyte stream velocity is higher in the center of the aperture than in the periphery due to boundary development. Some particles approaching the aperture obliquely travel close to the wall. These particles move slower than those that pass through the center of the aperture. The particles enter and leave the aperture boundaries through the zones of higher current density and may suffer shape distortions as a result of higher shear force near the wall resulting from the higher stream rate associated with the boundary layer. Errors may therefore result because pulse width measurements of larger particles moving in the center of the aperture might be quite similar to pulse width measurements of smaller particles moving near the aperture walls. For example, a particle traveling close to the wall of the aperture produces an `M`-shaped pulse. The pulse-height of this particle is significantly higher in comparison to the normal pulse due to a particle traveling through the center of the aperture. The resultant size distribution of a nearly mono-sized particle population is then strongly skewed toward higher volume. A true representation of the real size of the particle thus cannot be obtained. FIGS. 5 and 6 illustrate the error in the prior art due to the difference in the trajectory of the particle passing through the sensing zone. As the field lines are concentrated near the walls, a particle following a trajectory 20 which is close to the walls, gives a pulse 24 of higher magnitude in comparison to the pulse 26 associated particle that follows a trajectory 22 close to the axis of the aperture 1.
Besides the limitation on the smallest particle that can be measured with a given aperture, and the other drawbacks described above, the dynamic range of measurement is also limited. When a particle-free electrolyte passes through the aperture, the noise generated is mainly due to the electrical noise of the amplifier system. However, the noise increases greatly when a suspension of particles passes through the aperture. The absolute value of the noise increases with the increase in the size of particles. This happens partly because the particles moving just outside the aperture alter the conductivity gradient in the aperture. If the magnitude of this disturbance is greater than the signal due to the small particles, the measurement of small particles becomes impractical. Thus, the measurable range of sizes is limited, and it becomes difficult to distinguish between large and small particles in the same suspension.
Another limitation related to this phenomenon is an extended sensing zone error that occurs due to particles moving just outside the aperture. These external particles alter the conductivity gradient in the aperture. FIGS. 7 and 8 illustrate the error in the prior art due to the disturbance of extended sensing zone by particles outside the aperture. A large particle 28 located just outside the aperture 1 can significantly alter the signal on the electrodes 7 and 9, even before it enters the aperture 1. The peak 34 produced by this particle overshadows the peak 32 of a smaller particle 30 within the aperture 1 itself. Measurement of small particles in the presence of such interfering larger particles is thus impractical, when the magnitude of the disturbance is greater than the signal associated with the small particle. Thus, the range of overall sizes that can be measured becomes limited, and the ability to distinguish between large and small particles in the same suspension is hampered.
Disturbances depend upon the turbulence of the liquid at the boundary and the fringe effects of the electrical measuring fields. One phenomenon which should be mentioned as especially disturbing is that turbulence exists in the container which is located at the outlet of the channel in the through-flow direction. This turbulence recycles particles which have already been measured back into the region of the measuring field. Particles which have been recirculated in this manner re-trigger a change in the measured potential difference, thus falsifying the measurement result. It has already been proposed to provide a spatial limiting of the suspension in the channel. However, the equipment suitable for exploiting this technique is extremely complicated and correspondingly expensive. U.S. Pat. No. 4,161,690 addresses the recirculation problem by triggering sampling via the Coulter electrodes when the particle's passage through the middle of the channel is detected by a center electrode.
Thus, there is a requirement for an apparatus which can measure particle size and other properties more accurately than existing apparatus. If the particle measurements can be done more accurately and speedily the process for separation of different particles also improves. Counting, measuring, differentiating, separating and controlling the movement of particles is very critical in numerous industries like ceramics, cosmetics, explosives, powdered fuel, metal powder, abrasive, minerals, pharmaceutical, pigments, fillers, bio-technology and the like. Various parameters like volume, shape, rigidity, resistance and reactance have become extremely important in characterizing the properties of the particles and the fluid carrying the particles.